The present invention relates to a load driving device with a function of interrupting a circuit in the event of an overcurrent. In particular, the invention relates to a load driving device with a self-diagnosing function of detecting a failure in an overcurrent detecting function.
For example, in load driving devices for driving a load such as a lamp or a motor provided in a vehicle, a MOSFET as an electronic switch is provided between the battery and the load and the driving and the stop of driving of the load are controlled by turning on or off the MOSFET. They are provided with an overcurrent detecting function to protect circuit components of the load, MOSFET, and electrical wires when an overcurrent flows through the load by detecting it immediately.
FIG. 7 shows an exemplary circuit which is known as such a load driving device having an overcurrent detecting function.
As shown in FIG. 7, this load driving device is provided with a series circuit of a battery VB, a MOSFET T101 (hereinafter referred to simply as “FET”), and a load 101 such as a lamp or a motor.
A driver 102 is connected to the gate of the FET T101. When a drive signal is output from the driver 102, the FET T101 is turned on and the output voltage of the battery VB is applied to the load 101 to drive it.
The drain (voltage: V1) of the FET T101 is grounded via a series circuit of resistors R101 and R102, and the connecting point (voltage V4) of the resistors R101 and R102 is connected to the plus-side input terminal of a comparator CMP101. The source (voltage: V2) of the FET T101 is connected to the minus-side input terminal of the comparator CMP101. A term “1K” shown under the reference symbol R101 in FIG. 7 means that the resistance of the resistor R101 is 1 kΩ. And the same applies to the other resistors. That is, the resistance of the resistor R102 is 150 kΩ.
Resistors R103, R104, and R105 are provided parallel with the resistor R102, and their resistance values are set at 150 kΩ, 75 kΩ, and 37.5 kΩ, respectively. The resistors R103, R104, and R105 are grounded via FETs T102, T103, and T104, respectively.
Next, the operation of the above-configured load driving device will be described. When a drive signal is output from the driver 102, the FET T101 is turned on and a current ID flows along a path of the battery VB, the FET T101, and the load 101. The load 101 is driven being supplied with power in this manner. A voltage VDS (inter-electrode voltage) which is the difference between the voltage V1 of the drain (first electrode) and the voltage V2 of the source (second electrode) of the FET T101 is the product of the load current ID and an on-resistance Ron of the FET T101. That is, Equation (1) holds.VDS=V1-V2=Ron*ID  (1)
Since the resistance Ron is constant, the voltage VDS varies in proportion to the load current ID. Therefore, whether or not the load current ID is an overcurrent can be judged by monitoring the magnitude of the voltage VDS. This is done in the following manner. A voltage (V1-V4) across the resistor R101 is set as a judgment voltage. Occurrence of an overcurrent is detected through inversion of an output signal of the comparator CMP101 when the voltage VDS becomes higher than the judgment voltage (V1-V4), in other words, when the source voltage V2 of the FET T101 becomes lower than the voltage V4.
When all of the FETs T102-T104 are off, the voltage V4 is equal to a voltage obtained by dividing the voltage V1 between the resistors R101 and R102. Since the on-resistance of the FET T101 is usually about 5 mΩ and the load current ID is about 10 A in a normal state, the drain-source voltage VDS is equal to about 50 mV in a normal state. Based on this fact, the voltage V4 is set so that the voltage (V1-V4) becomes equal to about 100 mV. In the example of FIG. 7, when V1=14.5 V, the voltage (V1-V4) is given by the following Equation (2):
                                                                                          V                  ⁢                                                                          ⁢                  1                                -                                  V                  ⁢                                                                          ⁢                  4                                            =                              V                ⁢                                                                  ⁢                1                *                R                ⁢                                                                  ⁢                                  101                  /                                      (                                                                  R                        ⁢                                                                                                  ⁢                        101                                            +                                              R                        ⁢                                                                                                  ⁢                        102                                                              )                                                                                                                          =                              14.5                *                                  1                  /                                      (                                          1                      +                      150                                        )                                                                                                                          =                              96                ⁢                                                                  ⁢                                  (                  mV                  )                                                                                        (        2        )            
With the above setting, a relationship V4<V2 holds in an ordinary state. If an overcurrent occurs and the voltage V2 decreases to establish a relationship V4>V2, the output signal of the comparator CMP101 is inverted. The occurrence of the overcurrent is detected by detecting such inversion.
A rush current flows when driving of the load 101 is started. To prevent a rush current from being judged erroneously as an overcurrent, a control of changing the judgment voltage (V1-V4) is performed by supplying control signals A1-A3 to the FETs T102-T104. For example, if the FET T102 is turned on, the judgment voltage (V1-V4) is approximately doubled. If the FET T103 is turned on additionally, the judgment voltage (V1-V4) is approximately quadrupled. If the FET T104 is further turned on, the judgment voltage (V1-V4) is made about eight times greater than its original value. Controlling the FETs T102-T104 in this manner makes it possible to avoid such trouble that the circuit is interrupted erroneously due to a rush current or a normal variation of the load current ID.
In the above circuit configuration, if an on-failure occurs in at least one of the FETs T102-T104, that is, if at least one of the FETs T102-T104 is turned on for a certain reason though it should be kept off, the judgment voltage (V1-V4) is made two times or more greater than its normal-state value even in an ordinary state. In an ordinary operation, a control is performed so that all of the FETs T102-T104 are kept off. However, if the FET T102, for example, is turned on due to an on-failure, the judgment voltage (V1-V4) is made approximately two times greater than the value that would be obtained when the FET T102 were off.
If an overcurrent occurs in such a state, the overcurrent cannot be detected even if the voltage VDS is higher than the standard judgment value (i.e., the voltage VDS exceeds the judgment voltage that is set when all of the FETs T102-T104 are off). Proper protection cannot be given and the FET T101 may be broken or the lines may be burnt.
Even when an on-failure occurs in one or some of the FETs T102-T104, the failure cannot be found because no erroneous operation is caused as long as the load current ID that flows in an ordinary operation including a case of a rush current is concerned. The failure is found only after an overcurrent occurs actually due to short-circuiting or the like. That is, when an on-failure occurs in one or some of the FETs T102-T104, the failure does not appear until an overcurrent occurs actually due to short-circuiting or the like. Such a situation is not preferable from the fail-safe point of view.
As described above, the conventional load driving device is provided with not only the function of immediately detecting an overcurrent (current ID) flowing through the load but also the resistors R103-R105 and the FETs T102-T104 for preventing erroneous detection of a current flowing in a normal state such as a rush current. However, the conventional load driving device has the problem that the circuit cannot be protected reliably in the event of an overcurrent if an on-failure occurs in one or some of the FETs T102-T104.
The overcurrent protection circuit includes circuit elements and lines connecting those in addition to the circuit elements for the judgment voltage setting. If a failure occurs in those circuit elements and lines, the circuit protecting function may be lost. Since the loss of the circuit protecting function does not appear immediately, a failure continues to exist for a long time once it occurs. As a result, the circuit may not be protected in the event of an overcurrent accident though its probability of occurrence is low.